The present invention relates generally to permanent magnet electric motors for automobile applications and, more particularly, to a switching arrangement including both a resistor and a pulse width modulated (PWM) controlled switch that operate independently or in combination to control the speed of a permanent magnet electric motor.
For automotive engine-cooling fan motors, it is preferable to provide variable operating speeds so that the fan speed can be well matched to the cooling requirement. This ensures that the audible noises generated, and the electrical current drawn, are kept to the lowest achievable levels for the vehicle""s specific operating conditions.
Various methods of speed control have traditionally been used, including (1) the use of electrical resistors connected in series with the fan motor, or (2) connecting the motor via an electrical switching unit to provided a xe2x80x9cchoppedxe2x80x9d pulse width modulated current source. The advantages and disadvantages of these two conventional speed control methods are shown in the following table:
For new automotive applications, series resistors are now rarely specified because of their poor efficiency under controlled-speed operation, also because they typically only offer two speed choices.
The use of PWM electronics for motor control is becoming more widespread, but this system has a significant disadvantage. PWM-controlled motors typically generate an audible xe2x80x9cbuzzxe2x80x9d of the same frequency that is used for PWM switching. The noise is most problematic at motor start-up, where currents are momentarily high, and also at low speed operation where fan-related noise is not sufficient to mask the noise of the motor.
For some PWM applications, high-frequency switching is used which reduces the audible noise level but this requires more costly electronic switching components.
Accordingly, there is a need to provide a switching arrangement for motor speed control that combines the advantages of a series resistor (quiet motor start-up and quiet operation at low speed) with the advantages of PWM motor control (infinitely variable speed control with reasonable electrical efficiency).
An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing a motor speed control system including a permanent magnet direct current electric motor. A first switch is operatively associated with the motor. A resistive element is provided in series with the motor and the first switch. A second, pulse width modulated controlled, switch is operatively associated with the motor. A controller is operatively associated with the first and second switches for controlling the first and second switches independently and in combination to control current to the motor to operate the motor at various speeds.
In accordance with another aspect of the invention, a method of controlling speed of a permanent magnet direct current motor includes providing a first switch operatively associated with the motor. A resistive element is provided in series with the motor and the first switch. A second, pulse width modulated controlled, switch is operatively associated with the motor. The method includes controlling the switches independently and in combination to control current to the motor to operate the motor at various speeds.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.